1. Field of the Invention
The present invention relates to an improved microparticle system for use as an aid in making a paper product, i.e. paper or paperboard, with improved properties in the areas of retention, drainage, and sheet formation. More particularly, it pertains to a microparticle system comprising an acid colloid as a microparticle or an inorganic particulate material of the microparticle system.
2. Description of the Background Art
In the production of paper or paperboard, a dilute aqueous composition known as xe2x80x9cfurnishxe2x80x9d or xe2x80x9cstockxe2x80x9d is sprayed onto a moving mesh known as a xe2x80x9cwirexe2x80x9d. Solid components of this composition, such as cellulosic fibers and inorganic particulate filler material, are drained or filtered by the wire to form a paper sheet. The percentage of solid material retained on the wire is known as the xe2x80x9cfirst pass retentionxe2x80x9d of the papermaking process. Drainage, retention and formation (D/R/F) aids are used in the papermaking process.
Retention is believed to be a function of different mechanisms, such as filtration by mechanical entrainment, electrostatic attraction, and bridging between the fibers and the fillers in the furnish. Because both the cellulosic fibers and many common filler materials are negatively charged, they are mutually repellent. Generally, the only factor tending to enhance retention is mechanical entrainment. Therefore, a retention aid is generally used to improve retention of the fibers and fillers on the wire. The retention of fines and fillers is important to the papermaker to insure the capture of colloidally sized particles in the sheet. First pass retention (FPR) measures this ability of a retention program. Colloidal silica has been used in the past as a microparticle in a retention aid for alkaline fine paper. Silica has to be used properly in order to enhance the retention of fines and fillers by forming microflocs that capture colloidal material and allow the pulp slurry to dewater quickly.
Drainage relates to the rate of removal of water from the stock or furnish as the paper sheet is formed. Drainage usually refers to water removal that takes place before any pressing of the paper sheet subsequent to formation of the sheet. Thus, drainage aids are used to improve the overall efficiency of dewatering in the production of paper or paperboard.
Formation relates to the formation of the paper or paperboard sheet produced in the papermaking process. Formation is generally evaluated by the variance of light transmission within a paper sheet. A high variance is indicative of xe2x80x9cpoorxe2x80x9d formation and a low variance is generally indicative of xe2x80x9cgoodxe2x80x9d formation. Generally, as the retention level increases, the level of formation generally decreases from good formation to poor formation.
It can be appreciated that improvements in retention and drainage and in the formation properties of the paper or paperboard sheet are particularly desirable for several reasons, the most significant of which is productivity. Good retention and good drainage enable a paper machine to run faster and to reduce machine stoppage. Good sheet formation lessens the amount of paper wastage. These improvements are realized by the use of retention and drainage aids. Retention and drainage aids are additives that are used to flocculate the fine solid material present in the stock or furnish to improve these parameters in the papermaking process. The use of such additives is limited by the effect of flocculation on the paper sheet formation. If more retention aid is added so the size of the aggregates of the fine solid material is increased, then this generally results in variations in the density of the paper sheet which, as stated herein above, may result in what is referred to as xe2x80x9cpoorxe2x80x9d sheet formation. Over-flocculation can also affect drainage as it may eventually lead to holes in the sheet and/or to a subsequent loss of vacuum pressure in the later stages of dewatering during the papermaking process. Retention and drainage aids are generally added to the furnish in the wet-end of the paper machine, and generally are of three types, viz:
(a) single polymers;
(b) dual polymers; or
(c) a microparticle systems which may include flocculant and/or a coagulant.
A microparticle system generally gives the best result as a retention and drainage aid, and has been widely described in the prior art. In the past years, bentonite clay and colloidal silica have been used to improve drainage, retention, and formation.
Examples of publications describing microparticle systems include: EP-B-235, 893 wherein bentonite is used as the inorganic material in conjunction with a high molecular weight cationic polymer in a specified addition sequence; WO-A-94/26972 wherein a vinylamide polymer is disclosed for use in conjunction with one of various inorganic materials such as silica, bentonite, china clay, and organic materials; WO-A-97/16598 wherein kaolin is disclosed for use in conjunction with one of various cationic polymers; and EPO 805234 wherein bentonite, silica, or acrylate polymer is disclosed for use in conjunction with a cationic dispersion polymer.
U.S. Pat. Nos. 4,305,781 and 4,753,710 disclose the use of high molecular weight nonionic and ionic polymers in conjunction with bentonite clay to aid in dewatering and retention in papermaking. U.S. Pat. Nos. 4,388,150 and 4,385,961 teach the use of cationic starch and colloidal silica. U.S. Pat. Nos. 4,643,801 and 4,750,974 describe the use of cationic starch, anionic high molecular weight polymer, and colloidal silica in papermaking. U.S. Pat. No. 5,185,062 describes anionic polymer acting as a microparticle with a high molecular weight cationic flocculant. U.S. Pat. No. 5,167,766 teaches the use of charged organic polymeric microbeads as a microparticle in papermaking.
A microparticle system generally comprises a polymer flocculant with or without a cationic coagulant and a fine particulate material. The fine particulate material improves the efficiency of the flocculant and/or allows smaller, more uniform flocs to be produced.
The use of melamine-formaldehyde (MF) acid colloids for wet strength in paper is well known. Reference is made to TAPPI Monograph No. 29 xe2x80x9cWet Strength in Paper and Paperboardxe2x80x9d, C. S. Maxwell, J. P. Weidner, ed. U.S. Pat. No. 2,345,543 describes the preparation of stable melamine-formaldehyde acid colloids. U.S. Pat. No. 2,485,080 includes the incorporation of urea into the condensation products. U.S. Pat. Nos. 2,559,220 and 2,986,489 teach the use of these colloids to increase the wet strength of paper. U.S. Pat. No. 4,845,148 describes the use of amino-aldehyde acid colloid with acrylamide for increasing dry strength of paper. U.S. Pat. No. 5,286,347 describes the use of melamine formaldehyde colloid for pitch control in papermaking. U.S. Pat. No. 4,461,858 describes the use of polyvinyl alcoholxe2x80x94melamine formaldehyde colloid blends for wet-strength in paper. U.S. Pat. No. 4,009,706 teaches the use of melamine formaldehyde colloid and anionic high molecular weight polymer to flocculate raw sugar.
In spite of the several microparticle systems presently available for use in the paper mills to attain better runnability of the paper machine and/or to obtain a specific end use paper property, such as improved sheet formation for better printability, or improved surface strength, there remains a very real and substantial need for a microparticle system for improving the paper or paperboard by improving drainage and retention during the papermaking process and sheet formation properties in the formed sheet.
The present invention has met this above described need. The present invention relates to a microparticle system used as a retention and drainage aid in a papermaking process.
According to a first aspect of the present invention, there is a method of producing paper which comprises adding to a paper furnish a microparticle system as a retention and/or drainage aid which comprises a high molecular weight polymer flocculent and an inorganic particulate material comprising acid colloid comprised of an aqueous solution of a water soluble polymer or copolymer.
According to a second aspect of the present invention, there is a microparticle system which is added to a paper furnish as a retention and/or drainage aid, and which microparticle system comprises a high molecular weight polymer flocculant and an inorganic particulate material comprising an acid colloid comprised of an aqueous solution of a water-soluble polymer or copolymer.
According to a third aspect of the present invention, there is a paper or a paperboard product with improved properties in the area of retention, drainage and formation wherein the paper or paperboard product is made by adding a microparticle system to an aqueous cellulosic paper furnish, wherein the microparticle system comprises a high molecular weight polymer flocculant and an inorganic particulate material comprising an acid colloid comprised of an aqueous solution of a water-soluble polymer or copolymer.
A fourth aspect of the invention involves a process in which paper or paperboard is made by forming an aqueous cellulosic paper furnish, the steps comprising:
(a) adding to the thin stock flow of a paper furnish a high molecular weight polymer flocculant after a first shearing stage,
(b) at least after a second high shearing stage adding an inorganic particulate material comprising an acid colloid comprised of an aqueous solution of a water-soluble polymer or copolymer;
(c) draining the paper furnish to form a sheet; and
(d) drying the sheet.
In the several aspects of the invention and in a preferred embodiment, the acid colloid is comprised of an aqueous solution of a water-soluble polymer selected from the group consisting of melamine aldehyde, urea aldehyde, and melamine-urea aldehyde and the aldehyde is 
wherein R1 is selected from the group consisting of straight and branched C1-4 alkyl. The acid colloid is present in the stock or furnish in an amount ranging from about 0.0005% to about 0.5% by weight based on the dry weight of the solids in the stock or furnish.
Preferably, the aldehyde is formaldehyde and the acid colloid is melamine formaldehyde, which may be etherfied with a linear or branched alcohol.
The high molecular weight (HMW) polymer flocculant is present in an amount ranging from about 0.0025% to about 1.0% by weight based on the dry weight of the solids in the furnish. A high charge density cationic coagulant may be added to the stock or furnish prior to a first shearing stage or may in some instances be added prior to or after the addition of the acid colloid. Alternatively, the acid colloid and/or the flocculant could be added to the stock or furnish prior to the HMW flocculant and/or the coagulant and/or prior to the first shearing stage.